Nerf Jr. Foam Blaster: Attack of the Kleptons! is a Nerf video game that was released on November 9th, 1999 under the Nerf Jr. series.
Developed by Hasbro Interactive for the PC, it utilized a special light gun that resembled a regular Nerf blaster. This accessory was used to shoot the robotic Kleptons, the game's antagonists. (source: NERF Wiki)
Alternatively to the light gun you can right click to shoot, and press spacebar to reload.